snap-features-to-tin

Snap vector features to the faces of a triangulated irregular network (TIN).

Overview

Given a TIN representing a terrain mesh, this snaps 2D Point and LineString features to the faces of that mesh.

For Point features, this finds the triangle containing that point and linearly interpolates from the heights of the triangle's vertices to find the point's elevation.

For LineString features, this finds the elevation of every vertex using the same method as for Points, but also finds every intersection between each LineString segment and triangle edges. At each segment-edge intersection, a new vertex is added to the resulting LineString, so that every part of the LineString is attached to a face of the mesh.

Install

yarn add @kylebarron/snap-features-to-tin
# or
npm install @kylebarron/snap-features-to-tin

Usage

The single exported method is snapFeatures, which takes indices, positions, features, and bounds as arguments.

• indices: a flattened Array or TypedArray with triples of indices referring to positions. Each triple refers to a triangle face. So [1, 3, 4, ...] would mean that the second, fourth, and fifth (since zero-indexed) set of coordinates in positions constitute a triangle face.
• positions: a flattened Array or TypedArray with x, y, z coordinates. So [0.25, 0.5, 625, ...] would mean that the first position, i.e. 0 in indices, has position x=0.25, y=0.5, z=625 in the given coordinate space.
• features: an array of 2D Point and LineString GeoJSON features. Other geometry types will be silently skipped.
• bounds: an array of [minX, minY, maxX, maxY] to be used for clipping features. This is useful with features generated from vector tiles, since vector tiles usually have buffer outside the geographic extent it represents. But it's not possible to snap features outside the extent of the terrain mesh. Default: null.

Note that the coordinates of both features and positions need to be in the same coordinate system.

Example

import {snapFeatures} from '@kylebarron/snap-features-to-tin'
import {load} from '@loaders.gl/core';
import {TerrainLoader} from '@loaders.gl/terrain';

// array of GeoJSON features
const features = [...]

// Terrain mesh, generated from MARTINI
const terrain = await load(terrainImage, TerrainLoader);

// triples of position indices that make up the faces of the terrain
const indices = terrain.indices.value;

// x, y, z positions in space of each index
const positions = terrain.attributes.POSITION.value;

// bounding box to clip features to
// to turn off clipping, pass `bounds: null` (the default).
const bounds = [0, 0, 1, 1]

// Snap GeoJSON features to the mesh
// snappedFeatures is an Array of Features
const snappedFeatures = snapFeatures({indices, positions, features, bounds})